This invention relates to a metals and/or minerals recovery and waste treatment process that comprises a main separation stage and a waste sedimentation stage. The objective is to treat waste sediment from a metals and/or minerals processing facility in a novel and efficient manner.
Flocculation is the aggregation of single particles or small groups of particles, into multi-particle aggregates or ‘flocs’. The technique is used in many industrial processes to enhance solid-liquid separation.
In terms of definition it is necessary to make a distinction between coagulation and flocculation, since both involve particle aggregation but by different mechanisms. The process of coagulation involves particle aggregation by surface charge neutralization, thereby overcoming the repulsive potential barrier between particles. Particle surface charge is invariably negative and multivalent inorganic coagulants like lime, ferrous sulfate, ferric sulfate and alum are used to effect aggregation and form ‘coagula’. Flocculation in the strictest sense of the term is the aggregation of particles by long-chain polymers where particle surface charge may or may not be changed.
The term flocculation is derived from the Latin flocculus, which describes a tuft of wool, since this was likened to the “fluffy” aggregates that resulted from particle flocculation. The term flocculant is sometimes used as an adjective to describe the appearance of flocculated substances.
In general, the mineral processing stage of metals and/or minerals production involves a series of processing steps (see FIG. 1). Once the ore has been extracted from the ground, it requires upgrading (also referred to as beneficiation) to concentrate the metal or mineral values by separating it from the gangue. The separation process can be very simple or very complex involving many unit processes. Generally, one or more of the following unit processes are involved:
Comminution: (also referred to as size reduction)—comprised of two activities, this process involves crushing, followed by grinding in ball or rod mills, to reduce the size of the ore to a point that the minerals are liberated from one another and serves to prepare the material for physical and/or chemical separation.
Separation:—once the ore has been sized, the minerals are then separated by one or more of the following unit processes: size separation, which uses the difference in particle size of the different minerals (e.g., washing clay from sand on a screen); gravity separation, which uses the difference in density or specific gravity of the minerals—equipment used includes dense or heavy media, shaking tables, spirals, barrel washers, or jigs; electrical or magnetic separation, which uses those respective physical properties of the minerals; and froth flotation, which uses surface chemistry differences in the minerals.
Solid/Liquid Separation:—After the values and gangue minerals have been separated using one or more of the above unit processes, the resulting slurries (termed the concentrate and the tailings, respectively) need to be dewatered so they can be transported (in the case of concentrate) and disposed of in an environmentally acceptable manner (in the case of tailings). Types of solid/liquid separation equipment include thickeners, clarifiers, vacuum filters, pressure filters, multi-roll filters, and centrifuges.
Upon completion of the solid/liquid separation unit process of the mineral processing stage, three “product” streams emerge: (1) a recovered water stream that may, or may not, require additional clarification activities prior to being returned to the mining activities portion of the process; (2) a thickened concentrate stream that may, or may not, require additional processing (i.e., metallurgical processing) to produce the finished metal or mineral; and (3) a thickened tailings stream that may, or may not, require additional processing (e.g., pH adjustment) prior to being placed in a tailings pond for sedimentation (to remove and recover residual water) and disposal.
This invention relates to the “products” of the solid/liquid separation stage of mineral processing—i.e., the improved thickening of tailings (also referred to as sediment) and the improved clarity of water recovered for recycling (also referred to as supernatant).
Iron ore, an example of a ferrous metal, is produced from the extraction and processing of two ores: Hematite (Fe2O3) and/or Magnetite (Fe3O4). The ores are processed through a five-step process consisting of: (1) mining—the drilling and blasting to break the ore down into a suitable size; (2) crushing—the physical reduction in the size of the ore down to approximately 9 inches in diameter; (3) grinding—the further physical reduction in the size of the ore to liberate the fine mineral from the rock; (4) concentration—the separation of the iron values from the gangue via deslime thickeners (if the ore being processed is hematite) or via magnetic thickeners (if the ore being processed is magnetite) and the subsequent dewatering of the concentrated ore; and (5) pelletization—the formation of small, uniform-size balls “green balls” of iron ore using a chemical binder and the heat hardening of the balls to form hard iron ore pellets for use by steel manufacturing plants.
Sand and gravel, the unconsolidated granular materials resulting from the natural disintegration of rock or stone and an example of an industrial mineral, are typically mined in a wet condition by open pit excavation or dredging. After transportation to a processing plant the wet sand and gravel raw feed is stockpiled or emptied directly into a hopper where large cobbles and boulders are physically separated from the feed. From the hopper, the material is transported to fixed or vibrating screens where oversize material is separated from the smaller, marketable sizes. Oversize material may be used for erosion control, reclamation, or other uses, or it may be directed to a crusher for size reduction, to produce crushed aggregate, or to produce manufactured sands. The material that passes through the screen is fed into a battery of sizing screens, which generally consist of either horizontal or sloped—single or multi-deck—vibrating screens. Rotating trammel screens with water sprays are also used to process and wash wet sand and gravel. Screening separates the sand and gravel into different size ranges. After screening, the sized gravel is transported to stockpiles, storage bins, etc. for subsequent sale. The sand is freed from clay and organic impurities by log washers or rotary scrubbers. After scrubbing, the sand typically is sized by water classification. After classification, the sand is dewatered using screws, separatory cones, or hydro-separators. After processing, the sand is transported to storage bins for subsequent sale.
Flocculation of the waste stream (i.e. tailings) in order to improve the settling characteristics of an industrial process tailings pond has been proposed and practiced in the prior art. In flocculation, individual particles are united into rather loosely bound agglomerates or flocs. The degree of flocculation is controlled by the probability of collision between the particles and their tendency toward adhesion after collision. Agitation increases the probability of collision, and adhesion tendency is increased by the addition of a flocculant.
There have been numerous proposals in the literature to try to accelerate the sedimentation by flocculating the waste, and there have been proposals to improve the structure of the substantially solid waste sediment by adding sands or other materials to the waste. Examples of disclosures of such mineral recovery processes utilizing flocculants are U.S. Pat. Nos. 3,418,237, 3,622,087, 3,707,523, 4,194,969, 4,224,149, 4,251,363, 4,265,770, 4,342,653, 4,555,346, 4,690,752, 5,688,404, 6,077,441 and 6,039,189, which are each incorporated herein by reference.
Despite the numerous proposals to use flocculants, in practice it has been found that their use frequently is not cost effective. Even when a flocculant is used to promote sedimentation and the provision of a supernatant that can be recycled, the quality of the supernatant tends to be rather poor because the supernatant tends to be contaminated with unflocculated waste particles.
It is known within the metals and minerals processing industry that the utilization of above ground sedimentation columns (e.g., tubular metal tanks) allows for the formation of a useful depth of supernatant, provided that such columns have sufficient height. Unfortunately the volume of aqueous wastes which are generated in metals and/or mineral recovery processes can be so large that it is impracticable even to contemplate the provision of column separating tanks of this type.